Cold trap and cold trap regeneration method

ABSTRACT

A cold trap is provided with a cold panel provided in a pumping path such that the panel is exposed, a refrigerator thermally coupled to the cold panel and operative to cool the cold panel; and a controller configured, in a regeneration process for evaporating a gas frozen on the surface of the cold panel and discharging the gas outside using the vacuum pump, to control the refrigerator so as to raise the temperature of the cold panel to a temperature exceeding a non-liquefaction temperature range and to adjust a pressure in the pumping path at the temperature so that the gas frozen on the surface of the cold panel is evaporated without being melted, the non-liquefaction temperature range being a range in which it is guaranteed that a gas frozen on the surface of the cold trap is evaporated without being melted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cold trap and a method ofregenerating a cold trap.

2. Description of the Related Art

A cold trap is an apparatus for removing a gas from the environment byfreezing and capturing the gas on its surface. Normally, a coolingapparatus such as a system for supplying a cooling fluid or a cryogenicrefrigerator is provided in association with the cold trap so that thesurface of the cold trap is cooled to a cryogenic temperature.

For example, patent document No. 1 describes using a cold trap incombination with a dry pump such as a turbomolecular pump. A cold trapis provided in the interior of a vacuum chamber pumped primarily by thedry pump. Gases with a smaller molecular weight such as water vapor arefrozen and collected on the cold trap. Other gases with a relativelylarge molecular weight are pumped by the turbomolecular pump. Accordingto the document, a high degree of vacuum is obtained as a result. Aregeneration operation for melting and discharging the water moleculesthus frozen and collected is performed after performing a pumpingoperation for a predetermined period of time. A coolant heated by aheater heats the cold trap. This causes the frozen and collected watervapor to be liquefied and drained via a drain pipe provided immediatelybelow the cold trap.

-   [patent document No. 1] JP 9-313920

However, if the liquefied water or ice drops onto the turbomolecularpump, the turbomolecular pump may be adversely affected. In the worstcase, the turbomolecular pump may be damaged. In another aspect, timerequired for regeneration for a cold trap represents a downtime for avacuum chamber. Therefore, regeneration time is preferably as short aspossible.

SUMMARY OF THE INVENTION

In this background, a general purpose of the present invention is toprovide a cold trap and a method of regenerating a cold trap capable ofensuring that vacuum equipment such as a turbomolecular pump is lessaffected and reducing regeneration time.

An embodiment of the present invention relates to a cold trap. The coldtrap is provided in a pumping path connecting a volume subject topumping to a vacuum pump, causes a portion of a gas taken in from thevolume to the vacuum pump via the pumping path to be frozen on thesurface of the cold trap, and captures the gas accordingly. The coldtrap comprises: a cold panel provided in the pumping path such that thepanel is exposed; a refrigerator thermally coupled to the cold panel andoperative to cool the cold panel; and a controller configured, in aregeneration process for evaporating a gas frozen on the surface of thecold panel and discharging the gas outside using the vacuum pump, tocontrol the refrigerator so as to raise the temperature of the coldpanel to a temperature exceeding a non-liquefaction temperature rangeand to adjust, at the temperature exceeding a non-liquefactiontemperature range, a pressure in the pumping path so that the gas frozenon the surface of the cold panel is evaporated without being melted, thenon-liquefaction temperature range being a range in which it isguaranteed that a gas frozen on the surface of the cold trap isevaporated without being melted.

According to this embodiment, the cold trap is regenerated at arelatively high temperature exceeding the non-liquefaction temperature.Therefore, regeneration time is reduced. Since the ambient pressurearound the cold trap is controlled so that the ice captured on thesurface of the cold trap is evaporated without being melted, equipmentaround the cold trap is prevented from being adversely affected due toliquid water.

The controller may control the refrigerator so that the pressure in thepumping path does not exceed the pressure at the triple point of the gasfrozen on the cold panel.

The controller may control the refrigerator so that the pressure in thepumping path does not exceed a permissible inlet pressure of the vacuumpump.

When the pressure in the pumping path exceeds an upper limit pressure,the controller may return the pressure in the pumping path to a pressureequal to or below the upper limit pressure by cooling the cold panel.

When the pressure in the pumping path exceeds a permitted pressurerange, the controller may cool the cold panel to a standby temperatureselected from the non-liquefaction temperature range.

When the pressure in the pumping path falls below the permitted pressurerange at the standby temperature, the controller may raise thetemperature of the cold panel to a temperature exceeding thenon-liquefaction temperature range.

The controller may raise the temperature of the cold panel at a slowerrate at a temperature exceeding the non-liquefaction temperature rangethan at a temperature within the non-liquefaction temperature range.

The controller may raise the cold panel to a pressure determinationtemperature selected from the non-liquefaction temperature range anddetermine, after the temperature is raised, whether the pressure in thepumping path exceeds a reference pressure, and, when it is determinedthat the pressure in the pumping path exceeds the reference pressure,the controller may cool the cold panel to a temperature selected fromthe non-liquefaction temperature range, and, when it is determined thatthe pressure in the pumping path does not exceed the reference pressure,the controller may raise the temperature of the cold panel to atemperature exceeding the non-liquefaction temperature range.

The cold trap may further comprise a pressure sensor that includes inits measurement range the whole pressure range that could occur in thepumping path in the regeneration process, is provided to measure thepressure in the pumping path, and is connected to the controller so asto output a measured value to the controller. The controller may controlthe pressure in the pumping path based on the measured value from thepressure sensor.

Another embodiment of the present invention relates to a regenerationmethod. The cold trap regeneration method is for evaporating icecaptured on the surface of a cold trap and discharging the ice outside.The method comprises: raising the temperature of the cold trap to atemperature exceeding a non-liquefaction temperature range in which itis guaranteed that ice frozen on the surface of the cold trap isevaporated without being melted; and controlling an ambient pressurearound the cold trap at the temperature exceeding the non-liquefactiontemperature range so that the ice frozen on the surface of the cold trapis evaporated without being melted.

The controlling may exercise control so that the ambient pressure aroundthe cold trap does not exceed the pressure at the triple point of water.

The controlling includes discharging water vapor outside using aturbomolecular pump and controlling the ambient pressure around the coldtrap so as not to exceed a permissible inlet pressure of theturbomolecular pump.

When the ambient pressure around the cold trap exceeds an upper limitpressure, the controlling may return the ambient pressure to a pressureequal to or below the upper limit pressure by cooling the cold trap.

When the ambient pressure around the cold trap exceeds a permittedpressure range, the controlling may cool the cold trap to a standbytemperature selected from the non-liquefaction temperature range.

When the ambient pressure around the cold trap falls below the permittedpressure range at the standby temperature, the controlling may raise thetemperature of the cold trap to a temperature exceeding thenon-liquefaction temperature range.

The raising may raise the temperature of the cold trap at a slower rateat a temperature exceeding the non-liquefaction temperature range thanat a temperature within the non-liquefaction temperature range.

The raising may raise the temperature of the cold trap to a pressuredetermination temperature selected from the non-liquefaction temperaturerange and determine, after the temperature is raised, whether theambient pressure exceeds a reference pressure, and when it is determinedthat the ambient pressure exceeds the reference pressure, the ice may beallowed to sublimate at a temperature selected from the non-liquefactiontemperature range and is discharged outside, and, when it is determinedthat the ambient pressure does not exceed the reference pressure, thetemperature of the cold trap may be raised to a temperature exceedingthe non-liquefaction temperature range.

A single pressure sensor may be operative to measure the ambientpressure around the cold trap since the start of the raising through thecompletion of the controlling.

Still another embodiment of the present invention relates to aregeneration controller. The regeneration controller is adapted toperform a regeneration process for evaporating a gas frozen on thesurface of a cold trap and discharging the gas outside. The controlleris operative to raise the temperature of the cold trap to a temperatureexceeding a non-liquefaction temperature range and to control an ambientpressure around the cold trap at the temperature so that the gas frozenon the surface of the cold trap is evaporated without being melted, thenon-liquefaction temperature range being a range in which it isguaranteed that a gas frozen on the surface of the cold trap isevaporated without being melted.

Yet another embodiment of the present invention relates to aregeneration method. The cold trap regeneration method is forevaporating a gas frozen on the surface of a cold trap and dischargingthe gas outside. The method comprises: monitoring an ambient pressurearound the cold trap during regeneration; and temporarily cooling thecold trap when the ambient pressure thus monitored exceeds a permittedpressure range.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 schematically shows an evacuation system according to oneembodiment of the present invention;

FIG. 2 is a phase diagram of water;

FIG. 3 is a flowchart illustrating the regeneration process according tothe embodiment;

FIG. 4 is a flowchart illustrating the first temperature raising stepaccording to the embodiment;

FIG. 5 shows an exemplary temperature table according to the embodiment;

FIG. 6 is a flowchart illustrating the second temperature raising stepaccording to the embodiment;

FIG. 7 shows an exemplary pressure table according to the embodiment;

FIG. 8 shows an exemplary temperature table according to the embodiment;

FIG. 9 shows an exemplary final output table according to theembodiment;

FIG. 10 is a flowchart illustrating the high-temperature pumping stepaccording to the embodiment;

FIG. 11 is a flowchart illustrating the cool-down step according to theembodiment;

FIG. 12 shows the time variation of the temperature and pressure in theregeneration process according to the embodiment; and

FIG. 13 is a graph showing another example of the time variation of thetemperature and pressure in the regeneration process according to theembodiment.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described by reference to the preferredembodiments. This does not intend to limit the scope of the presentinvention, but to exemplify the invention.

In one embodiment of the present invention, regeneration time is reducedby performing the regeneration of a cold trap at a high temperature. Ina typical regeneration method, regeneration is performed at a lowtemperature (e.g., below 260 K) in order to evaporate a gas frozen onthe surface of a cold trap, bypassing a liquid phase. Liquid phase isavoided in order to prevent vacuum equipment (e.g. a turbomolecularpump) in the neighborhood from being affected. In contrast, according tothe embodiment, the ambient pressure around a cold trap beingregenerated is adjusted to fall within a pressure range in which afrozen gas is evaporated without being melted. For this reason,regeneration can be performed at a high temperature while avoiding aliquid phase of the captured gas.

Further, in a typical regeneration method, once a cold trap is heated toa regeneration temperature, the regeneration temperature is maintaineduntil pumping is completed. Nothing in the related art teaches atechnical idea of cooling a cold trap during regeneration. In theembodiment, however, a cold trap is allowed to be cooled temporarily sothat the ambient pressure around the cold trap does not reach a pressureat which the frozen gas can be melted. In this way, the rate ofsublimation of the gas from the cold trap is reduced and the ambientpressure is prevented from being increased. We have experimentallyverified that regeneration time is considerably reduced as compared withthe related-art low-temperature regeneration by performinghigh-temperature regeneration in combination with temporary cooling of acold trap.

In one embodiment of the present invention, a cold trap is provided witha controller for controlling a regeneration process. The controllerraises the temperature of the cold trap to a temperature exceeding anon-liquefaction temperature range. With this, the gas frozen on thesurface of the cold trap is evaporated again and discharged outside. Inthis process, the controller controls the ambient pressure so that thegas frozen on the surface of the cold trap is evaporated without beingmelted. The non-liquefaction temperature range is a temperature range inwhich it is guaranteed that the gas frozen on the surface of the coldtrap is evaporated without being melted. For example, thenon-liquefaction temperature range is determined based on the phasediagram of the gas.

The controller controls the ambient pressure around the cold trap so asnot to exceed a limit pressure. The limit pressure may be defined as apressure at the triple point of the gas to be pumped. In the case ofdischarging the re-evaporated gas outside by means of a turbomolecularpump, the limit pressure may be equal to the permissible inlet pressureof the turbomolecular pump. A permissible inlet pressure is defined inthe specification of a turbomolecular pump and denotes a maximumpressure permitted at the inlet of a turbomolecular pump. In order toensure that the ambient pressure around the cold trap does not exceedthe limit pressure, the controller controls the temperature of the coldtrap so that the ambient pressure is maintained in a permitted pressurerange defined by a predetermined upper limit pressure lower than thelimit pressure and a predetermined lower limit pressure lower than theupper limit pressure.

In one embodiment, the controller performs a regeneration process byperforming a temperature raising step of raising the temperature of thecold trap to a predetermined regeneration temperature, a dischargingstep of discharging the gas captured on the cold trap outside, and acool-down step of re-cooling the cold trap in the stated order. Thecontroller selects one of a plurality of predefined regenerationtemperatures. The first regeneration temperature exceeding thenon-liquefaction temperature range and the second regenerationtemperature selected within the non-liquefaction temperature range maybe at least established. For example, the first regeneration temperaturemay be set to be lower than the upper temperature limit of therefrigerator. The second regeneration temperature may be set to be equalto or slightly lower than the upper limit temperature of thenon-liquefaction temperature range.

In the temperature raising step, the controller may raise thetemperature of the cold trap to a pressure determination temperature soas to determine whether or not the ambient pressure around the cold trapexceeds a reference pressure. For example, the pressure determinationtemperature is selected from the non-liquefaction temperature range. Thepressure determination temperature may be equal to the secondregeneration temperature. For example, the reference pressure is set tobe lower than the above-mentioned limit pressure. The reference pressuremay be set to be equal to, for example, the upper limit pressure of thepermitted pressure range. The controller may perform the dischargingstep at a temperature selected from the non-liquefaction temperaturerange (e.g., at the second regeneration temperature) when it isdetermined that the ambient pressure around the cold trap exceeds thereference pressure and may perform the discharging step at a temperatureexceeding the non-liquefaction temperature range (e.g., at the firstregeneration temperature) when it is determined that the ambienttemperature around the cold trap does not exceed the reference pressure.

In the temperature raising step, the controller may raise thetemperature at a slower rate at a temperature exceeding thenon-liquefaction temperature range than in the non-liquefactiontemperature range. For example, the rate of raising the temperature fromthe pressure determination temperature to the first regenerationtemperature may be slower than the rate of raising the temperature tothe pressure determination temperature. The controller may decrease thetemperature raising rate in steps or continuously as the temperature israised. The controller may monitor the ambient pressure around the coldtrap and decrease the temperature raising rate when the ambient pressureexceeds a predetermined pressure. In this way, the ambient pressurearound the cold trap is prevented from being increased abruptly in thetemperature raising step. As a result, the gas frozen on the cold trapis prevented from being melted.

When the ambient pressure around the cold trap exceeds the upper limitpressure in the discharging step, the controller may cool the cold trapso as to return the ambient pressure to a pressure below the upper limitpressure. For example, the controller may cool the cold trap to astandby temperature when the ambient pressure around the cold trapexceeds the permitted pressure range. The controller may raise thetemperature of the cold trap to a regeneration temperature (e.g., thefirst regeneration temperature) exceeding the non-liquefactiontemperature range when the ambient pressure around the cold trap fallsbelow the permitted pressure range at the standby temperature. In thisway, the controller is permitted to temporarily cool the cold trap inthe discharging step so as to maintain the ambient pressure around thecold trap within the permitted pressure range.

For example, the standby temperature is selected from thenon-liquefaction temperature range. The standby temperature may be setto be equal to the second regeneration temperature. By ensuring that thestandby temperature is low, the ambient pressure around the cold trap ispromptly returned to the permitted pressure range. If, however, theupper limit value of the permitted pressure range of the ambientpressure around the cold trap is set to be sufficiently lower than thetriple point pressure of the gas frozen on the cold trap, the standbytemperature exceeding non-liquefaction temperature range may beselected. This is due to the fact that, if the upper limit value of thepermitted pressure range is sufficiently lower than the triple pointpressure, the gas frozen on the cold trap is not considered to be meltedeven when the ambient pressure exceeds the permitted pressure range. Bymaintaining the temperature of the cold trap at a high level,regeneration time is reduced accordingly. In this respect, it ispreferable to optimally set the standby temperature by considering thecontrollability of the ambient pressure around the cold trap and animpact on the regeneration time.

For example, the cold trap according to the embodiment is an in-linecold trap. In other words, the cold trap is provided in a pumping pathconnecting the volume subject to pumping (e.g., a vacuum chamber) to avacuum pump. The cold trap causes a portion of a gas taken in from thevolume to the vacuum pump via the pumping path to be frozen on itssurface and captures the gas accordingly. In this case, the pressure inthe pumping path represents the ambient pressure around the cold trap.The vacuum pump may be implemented by a turbomolecular pump or adiffusion pump. In this case, the operating temperature of the cold trapis primarily set to capture water vapor. The turbomolecular pump may notonly be operated for pumping. The pump may also be used whileregenerating the cold trap in order to discharge the re-evaporated gasoutside.

In one embodiment, the cold trap may be provided with a pressure sensorfor measuring the ambient pressure. For example, the pressure sensor isprovided to measure the pressure in the pumping path mentioned above.The pressure sensor may include in its measurement range the wholepressure range that could occur during the regeneration process. Thesensor may be a crystal gauge. More specifically, the pressure sensormay include in its measurement range both a pressure for determining thecompletion of the discharging step and the above-mentioned upper limitpressure. The pressure sensor may monitor the ambient pressure aroundthe cold trap from the beginning of the regeneration process through thecompletion thereof. The controller may adjust the temperature of thecold trap based on a measured value from the pressure sensor.

The cold trap according to the embodiment is provided with a cold paneloperative to capture a gas on its surface, and a refrigerator thermallycoupled to the cold panel and operative to cool the cold panel. Therefrigerator according to the embodiment is capable of a normaloperation (hereinafter, sometimes referred to as normal rotationoperation) for cooling the cold panel and a reverse rotation operationfor heating the cold panel. In a normal operation, the refrigeratorproduces refrigeration by a heat cycle whereby the operating gas takeninside is expanded and discharged. In a reverse rotation operation, heatis generated by a heat cycle produced by reversing the heat cycle innormal operation. The controller adjusts the temperature of the coldpanel by switching between normal operation and reverse rotationoperation of the refrigerator. The controller may adjust the temperatureof the cold panel by controlling the frequency of heat cycles in normaloperation and/or heat cycles in reverse rotation operation.

In one embodiment, the controller determines the operation state of therefrigerator so as to accommodate the temperature of the cold panel andthe ambient pressure within a permitted range. However, if the ambientpressure around the cold panel deviates from the permitted pressurerange, or it is predicted that the pressure will deviate, the controllermay determine the operation state of the refrigerator giving priority toaccommodating the ambient pressure around the cold panel within thepermitted pressure range rather than accommodating the temperature ofthe cold panel within the permitted temperature range. For example, ifthe ambient pressure around the cold panel exceeds the permittedpressure range in the discharging step, the controller may switch theoperation state of refrigerator from reverse rotation operation tonormal operation so as to cool the cold panel to a temperature lowerthan the permitted temperature range. Alternatively, if the ambientpressure around the cold panel falls below the permitted pressure range,the controller may switch the operation state of the refrigerator fromnormal operation to reverse rotation operation so as to heat the coldpanel to a temperature higher than the permitted temperature range.

A detailed description will now be given of the best mode of carryingout the invention with reference to the drawings. FIG. 1 schematicallyshows an evacuation system according to one embodiment of the presentinvention. The evacuation system is provided with a cold trap 10 and aturbomolecular pump 12. The turbomolecular pump 12 is connected to avacuum chamber 16 of a vacuum processing apparatus via a pumping path14. The cold trap 10 is provided in front of the turbomolecular pump 12in the pumping path 14. The cold trap 10 is provided above theturbomolecular pump 12 in the vertical direction.

Further, a gate valve 18 for shielding the evacuation system from thevacuum chamber 16 is provided in the pumping path 14. The gate valve 18is provided between the opening of the vacuum chamber 16 and the coldtrap 10. By opening the gate valve 18, the evacuation systemcommunicates with the vacuum chamber 16 for pumping. By closing the gatevalve 18, the evacuation system is shielded from the vacuum chamber 16.When regenerating the cold trap 10, the gate valve 18 is normallyclosed. The gate valve 18 may constitute the evacuation system.Alternatively, the gate valve may be provided at the opening of thevacuum chamber 16 as part of the vacuum processing apparatus.

The cold trap 10 comprises a cold panel 20, a refrigerator 22, and acontroller 24. The entirety of the cold panel 20 is exposed to thepumping path 14 and causes a portion of the gas flowing in the pumpingpath 14 to be frozen on its surface and captures the gas accordingly.The cold panel 20 is provided in a plane perpendicular to the directionof gas flow in the pumping path 14 (the vertical direction in FIG. 1).The area of the cold panel 20 projected along the direction of gas flowin the pumping path 14 is set so as to occupy the majority of the crosssectional area perpendicular to the direction of gas flow.

The cold panel 20 is a louver having a plurality of metallic vanes. Thevanes are formed like sides of truncated cones having differentdiameters and are concentrically arranged. The cold panel 20 may be in achevron formation or may form a lattice or other shape.

The cold panel 20 is thermally coupled to a cooling stage 28 of therefrigerator 22 by a heat transfer member 26 projecting from thecircumference of the panel. An opening is formed in the pumping path 14at a position coinciding with the heat transfer member 26. Mounted inthe opening is a coupling housing 32 that accommodates the heat transfermember 26 and connects the pumping path 14 to a refrigerator housing 30.The coupling housing 32 hermetically connects the interior space of thepumping path 14 to the interior space of the refrigerator housing 30.This results in the internal pressure of the refrigerator housing 30being equal to the pressure in the pumping path 14.

The refrigerator 22 is a Gifford-McMahon refrigerator (so-called a GMrefrigerator). The refrigerator 22 is a single-stage refrigerator and isprovided with the cooling stage 28, a cylinder 34, and a refrigeratormotor 36. The refrigerator stage 28 is mounted at one end of thecylinder 34. The refrigerator motor 36 is provided at the other end ofthe cylinder 34. A displacer (not shown) is built in the cylinder 34 anda regenerator is built in the displacer. The refrigerator motor 36 isconnected to the displacer so that the displacer can make a reciprocalmovement inside the cylinder 34. The refrigerator motor 36 is alsoconnected to a movable valve (not shown) provided inside therefrigerator 22 so as to drive the valve into normal and reverserotation.

A compressor (not shown) is connected to the refrigerator 22 via ahigh-pressure pipe and a low-pressure pipe. The refrigerator 22 producesrefrigeration in the cooling stage 28 and the cold panel 20 by repeatingheat cycles whereby a high-pressure operating gas (e.g., helium)supplied from the compressor is expanded inside the refrigerator 22 andthen discharged. The refrigerator motor 36 rotates the movable valve ina predetermined direction so as to achieve the heat cycle. Thecompressor collects the operating gas discharged from the refrigerator22 and increases its pressure before supplying it the refrigerator 22again. By allowing the refrigerator motor 36 to rotate the movable valvein a reverse direction, a heat cycle produced by reversing the aboveheat cycle is achieved so that the cooling stage 28 and the cold panel20 are heated. Instead of or in addition to reverse rotation operationof the refrigerator 22, the cooling stage 28 or the cold panel 20 may beheated by using a heating means such as a heater.

A temperature sensor 38 is provided in the cooling stage 28 of therefrigerator 22. The temperature sensor 38 periodically measures thetemperature of the cooling stage 28 and outputs a signal indicating themeasured temperature to the controller 24. The temperature sensor 38 isconnected to the controller 24 so that an output of the sensor can becommunicated to the controller 24. The cooling stage 28 and the coldpanel 20 are formed as one piece thermally. Therefore, the measuredtemperature from the temperature sensor 28 indicates the temperature ofthe cold panel 20. The temperature sensor 38 may be provided in the coldpanel 20 or in the transfer member 26.

A pressure sensor 40 is provided inside the refrigerator housing 30. Thepressure sensor 40 periodically measures the internal pressure of therefrigerator housing 30, i.e., the pressure in the pumping path 14, andoutputs a signal indicating the measured pressure to the controller 24.The pressure sensor 40 is connected to the controller 24 so that anoutput of the sensor can be communicated to the controller 24. Thepressure sensor 40 may measure the pressure only during a regenerationprocess of the cold trap 10 and output the result to the controller 24.The measured value from the pressure sensor 40 indicates the pressurearound the cold panel 20, i.e., the ambient pressure. The pressuresensor 40 may be provided inside the housing 32 or in the pumping path14.

The pressure sensor 40 has a wide measurement range extending fromatmospheric pressure to about 0 Pa. Desirably, the sensor 40 includes inits measurement range at least a pressure range that could occur duringa regeneration process. Desirably, the pressure sensor 40 is a pressuresensor at least capable of measuring a pressure occurring when the gasflow in the pumping path 14 is a viscous flow. The pressure sensor 40may be a sensor also capable of measuring a pressure irrespective ofwhether the gas flow in the pumping path 14 is a viscous flow or amolecular flow. Generally, the gas flow will be a viscous flow if thepressure in the pumping path 14 is higher than several Pa. The gas flowwill be a molecular flow if the pressure is smaller than 10⁻¹-10⁻² Pa.In the embodiment, it is preferable to use, for example, a crystal gaugeas a sensor that meets the requirement. A crystal gauge is a sensor formeasuring a pressure by using a phenomenon whereby vibration resistanceof a crystal oscillator varies with pressure. A Baratron vacuum gage maybe used alternatively. In a typical, related-art cold trap, there is notprovided a pressure sensor, or a pressure sensor capable of measuringonly the pressure of a molecular flow is used (e.g., a T/C gauge).

The controller 24 is formed as a microprocessor including a CPU. Inaddition to the CPU, the controller 24 is provided with a ROM forstoring programs, a RAM for temporarily storing data, an input andoutput port, and a communication port. The controller 24 is connected tothe controller of the vacuum processing apparatus and capable ofcommunicating therewith. The controller 24 is also capable of executingproper control in accordance with an instruction from the controller ofthe vacuum processing apparatus. The controller 24 controls therefrigerator 22 based on the measured value fed from the temperaturesensor 38 and the pressure sensor 40. The controller 24 is connected tothe refrigerator motor 36 of the refrigerator 22 and capable ofcommunicating therewith. An inverter (not shown) is provided between thecontroller 24 and the refrigerator motor 36. The revolution of therefrigerator motor 36 is controlled by supplying an instruction from thecontroller 24 to the inverter. By changing the revolution of therefrigerator motor 36, the frequency of heat cycles in the refrigerator22 is changed so that the temperature of the cooling stage 28 and thecold panel 20 varies.

The evacuation system shown in FIG. 1 alternately repeats a pumpingprocess and a regeneration process. In a pumping process, the vacuumchamber 16 is evacuated to increase the degree of vacuum to a desiredlevel by opening the gate valve 18 and operating the turbomolecular pump12. In this process, the cold trap 10 is cooled to a temperature (e.g.,100 K) capable of capturing water vapor flowing in the pumping path 14.Normally, the pumping speed of the turbomolecular pump 12 for pumpingwater vapor is relatively low. However, a larger pumping speed isachieved by using the cold trap 10 in combination.

In a pumping process, the controller 24 controls the refrigerator motor36 based on the measured temperature from the temperature sensor 38 sothat the temperature of the cold panel 20 matches a target temperature(e.g., 100 K). For example, the controller 24 determines the revolutionof the refrigerator motor 36 so that an error between the measuredtemperature from the temperature sensor 38 and the target temperature isminimized. For example, the controller 24 increases the revolution ofthe refrigerator motor 36 when the measured temperature exceeds thetarget temperature and decreases the revolution of the refrigeratormotor 36 when the measured temperature falls below the targettemperature. In this way, the temperature of the cold panel 20 ismaintained at the target temperature.

As the pumping process is continued, the frozen gas is collected on thecold trap 10. What mainly occurs in the embodiment is that the amount ofice formed by solidified water vapor is mainly increased. Thus, for thepurpose of discharging the ice thus collected outside, the cold trap 10is regenerated after an elapse of a predetermined time since the startof the pumping process. Regeneration is normally performed by closingthe gate valve 18 and isolating the cold trap 10 from the vacuum chamber16 accordingly. The temperature of the cold trap 10 is raised to aregeneration temperature higher than the cold trap temperature occurringduring the pumping process so as to re-evaporate the gas frozen on thesurface. The re-evaporated gas is discharged outside by operating theturbomolecular pump 12. It will also be possible to provide a vacuumpump other than the turbomolecular pump for the purpose of regenerationand to use the vacuum pump to discharge the gas outside.

In the regeneration process according to the embodiment, the pressure inthe pumping path 14 is monitored and controlled not to exceed the limitpressure from the start of the regeneration process through thecompletion thereof. For example, the limit pressure may be equal to thetriple point pressure. In the embodiment, the cold trap 10 primarilypumps moisture. Therefore, the triple point pressure of water may be setas the limit pressure. FIG. 2 is a phase diagram of water. FIG. 2 showsthat the triple point pressure of water is 611 Pa. Accordingly, thelimit pressure may be set to 611 Pa. In this way, the pressure duringregeneration is controlled to be equal to or below the triple pointpressure so that the ice is allowed to sublimate directly. Since aliquid phase is not allowed, it is ensured that water or ice does notdrop onto the turbomolecular pump 12 provided immediately below the coldtrap 10. Accordingly, the turbomolecular pump 12 is prevented from beingdamaged. Another potential concern is that liquid water may produce ahazardous composition as a result of chemical reaction with anothercaptured gas. By preventing liquid water from being produced, thelikelihood of producing a hazardous composition is also reduced.

The limit pressure may alternatively be set to the permissible inletpressure of the turbomolecular pump 12. For example, the permissibleinlet pressure of the turbomolecular pump 12 is about several Pa or onthe order of 10-100 Pa. The permissible inlet pressure may be 100-200Pa. In the embodiment, the permissible inlet pressure is, for example,100 Pa. If the permissible inlet pressure of the turbomolecular pump 12is lower than the triple point pressure of the gas to be pumped by thecold trap 10, it is preferable to set the limit pressure to thepermissible inlet pressure. In this way, the interior of theturbomolecular pump 12 is prevented from being overheated during aregeneration process.

FIG. 3 is a flowchart illustrating the regeneration process according tothe embodiment. The process shown in FIG. 3 is started when aninstruction for starting a regeneration process is fed to the controller24. The instruction for staring a regeneration process is generated by,for example, the controller of the vacuum processing apparatus and fedto the controller 24. Alternatively, the instruction for starting aregeneration process may be directly fed from an input interfaceprovided in association with the controller 24 to the controller 24. Thecontroller 24 may be configured to start the regeneration process afteran elapse of predetermined delay time since the reception of theinstruction for starting a regeneration process.

The controller 24 performs the first temperature raising step (S10). Inthe first temperature raising step, the controller 24 raises thetemperature of the cold panel 20 to a pressure determinationtemperature. The pressure determination temperature is a temperatureselected from the non-liquefaction temperature range and is predefinedby and stored in the controller 24. In the embodiment, the pressuredetermination temperature is set to, for example, 260 K. The firsttemperature raising step will be described later in further detail withreference to FIGS. 4 and 5.

The non-liquefaction temperature range of water can be defined byreferring to the phase diagram shown in FIG. 2. The non-liquefactiontemperature range of water is a temperature range in which it isguaranteed that ice on the surface of the cold panel 20 evaporates bysublimation in a pressure range that occur during a regenerationprocess. The non-liquefaction temperature range of water may be definedas a temperature range equal to or below 270 K based on the phasediagram of water. Since the gradient of the melting curve is negative inthe case of water as shown in FIG. 2, it is preferable to define thenon-liquefaction temperature range as being a range below a temperatureslightly lower than 273.16 K, which is the triple point temperature. Inthe case of a gas with a positive gradient of the melting curve, thenon-liquefaction temperature range may be defined as a range below thetriple point temperature.

Following the first temperature raising step, the controller 24 performsthe first pressure determination step (S12). The controller 24determines whether the ambient pressure around the cold trap occurringafter the first temperature raising step is higher than the referencepressure. The reference pressure is predefined by and stored in thecontroller 24. The reference pressure may be set to be equal to thelimit pressure mentioned above. Alternatively, the reference pressuremay be set to lower than the limit pressure by an appropriate margin. Inthe embodiment, the reference pressure is set to, for example, 100 Pa,which is the permissible inlet pressure of the turbomolecular pump 12.

If it is determined that the ambient pressure around the cold trapoccurring after the first temperature raising step is higher than thereference pressure (Yes in S12), the controller 24 performs alow-temperature pumping step (S14). In other words, the controller 24continues the regeneration process at the second regenerationtemperature selected from the non-liquefaction temperature range. Inthis case, the ambient pressure around the cold trap is considerablyhigh so that priority is given to preventing liquid water from beingproduced rather than to reducing regeneration time, by maintaining thenon-liquefaction temperature range. For example, the second regenerationtemperature may be set to be equal to the pressure determinationtemperature. Thus, the second regeneration temperature is set to 260 Kin the embodiment. In order to reduce regeneration time, the secondregeneration temperature is preferably set to the upper limittemperature of the non-liquefaction temperature range or a temperaturelower than the upper limit temperature by a predetermined margin. Themargin may be appropriately set in consideration of temperature controlerror or heat transfer property, so as not to deviate from thenon-liquefaction temperature range at any position on the cold panel 20.When the low-temperature pumping step is completed, the controller 24performs a cool-down step (S24), thereby completing the regenerationprocess.

If it is determined that the ambient pressure around the cold trapoccurring after the first temperature raising step is equal to or lowerthan the reference pressure (No in S12), the controller 24 performs thesecond temperature raising step (S16). In the second temperature raisingstep, the controller raises the temperature of the cold panel 20 fromthe pressure determination temperature to the first regenerationtemperature. The first regeneration temperature is a temperature thatexceeds the non-liquefaction temperature range and is predefined by andstored in the controller 24. In the embodiment, the first regenerationtemperature is set to, for example, 320 K. It is preferable that thefirst regeneration temperature be lower than the upper temperature limitof the refrigerator 22. Desirably, the first regeneration temperature isset to a temperature lower than the refrigerator's upper temperaturelimit by a predetermined margin. The margin may be appropriately set inconsideration of temperature control error or heat transfer property, soas not to exceed the upper temperature limit at any position in therefrigerator 22. The second temperature raising step will be describedlater in further detail with reference to FIGS. 6 through 9.

The controller 24 may raise the temperature at a higher rate in thefirst temperature raising step than in the second temperature raisingstep. By raising the temperature at a higher rate in the firsttemperature raising step, reduction in regeneration time is achieved. Byraising the temperature slowly in the second temperature raising step,abrupt increase in the ambient pressure around the cold trap isprevented. More specifically, the controller 24 decreases the heat cyclefrequency of the refrigerator 22 in the second temperature raising stepas compared to the first temperature raising step. In other words, thecontroller 24 decreases the revolution of the refrigerator motor 36 inthe second temperature raising step as compared to the first temperatureraising step.

Following the second temperature raising step, the controller 24performs the second pressure determination process (S18). The controller24 determines whether the ambient pressure around the cold trapoccurring after the second temperature raising step is lower than apumping completion pressure. The pumping completion pressure ispredefined by and stored in the controller 24. In the embodiment, thepumping completion pressure is set to, for example, 5 Pa. The pumpingcompletion pressure is a pressure at which it is considered that the gasstored in the cold trap 10 is completely pumped and can be appropriatelyset empirically or experimentally. In order to measure the pressure witha good precision, it is preferable to set the pumping completionpressure at a value larger than the minimum measurable pressure of thepressure sensor 40 used.

If it is determined that the ambient pressure around the cold trapoccurring after the second temperature raising step is lower than thepumping completion pressure (Yes in S18), the controller 24 stands byfor a predetermined period of time at the first regeneration temperature(S20), performs a cool-down step (S24), and completes the regenerationprocess. If the ambient pressure is lower than the pumping completionpressure, it means that the gas is completed discharged. Therefore, theregeneration process may be terminated. By standing by for apredetermined period of time at the first regeneration temperature,whatever ice that remains on the cold panel 20 is discharged. In theembodiment, the controller 24 stands by for several to ten minutes. Ifreduction of regeneration time is given priority, the standby time maynot be provided. In the cool-down step, the controller 24 cools the coldpanel 20 to a panel target temperature defined for the pumping process(e.g., 100 K). The cool-down step will be described later in furtherdetail with reference to FIG. 11.

If it is determined that the ambient pressure abound the cold trapoccurring after the second temperature raising step is higher than thepumping completion pressure (No in S18), the controller 24 performs ahigh-temperature pumping step (S22). In the high-temperature pumpingstep, the controller 24 continues the regeneration process at the firstregeneration temperature higher than the non-liquefaction temperaturerange. However, if the ambient pressure around the cold trap exceeds thepermitted range, the ambient pressure is returned to the permitted rangeby temporarily cooling the cold panel 20. The high-temperature pumpingstep will be described later in further detail with reference to FIG.10. When the high-temperature pumping step is completed, the controller24 performs a cool-down step (S24) and completes the regenerationprocess.

FIG. 4 is a flowchart illustrating the first temperature raising step(S10 of FIG. 3) according to the embodiment. The controller 24determines whether the cold panel temperature T exceeds the pressuredetermination temperature (e.g., 260 K) (S30). If it is determined thatthe cold panel temperature T exceeds the pressure determinationtemperature (No in S30), the controller 24 terminates the firsttemperature raising step and performs the first pressure determinationprocess (S12 of FIG. 3). In other words, the first temperature raisingstep is omitted. For example, when the regeneration process is startedwhen a long period of time has elapsed since the completion of thepumping operation of the evacuation system, the cold panel temperaturemay rise naturally to a level exceeding the pressure determinationtemperature. In such a case, regeneration time can be reduced byomitting the first temperature raising step.

If it is determined that the cold panel temperature T is equal to orbelow the pressure determination temperature (Y in S30), the controller24 starts temperature control wherein the pressure determinationtemperature is the target temperature (S32). Such temperature controlmay be referred to as low-temperature control hereinafter. Since thepressure determination temperature is 260 K in the embodiment, suchtemperature control may be referred to as 260 K temperature control. Thecontroller 24 determines whether the cold panel temperature T hasreached 260 K since the start of 260 K temperature control (S34). If itis determined that the cold panel temperature T has not reached 260 K(No in S34), the controller 24 determines whether the cold paneltemperature T has reached 260 K at a subsequent point of time in thecontrol schedule (S34).

If it is determined that the cold panel temperature T is equal to orhigher than 260 K (Yes in S34), the controller 24 continues 260 Ktemperature control for a predetermined standby time (S36). The standbytime is provided because it is considered that an increase in theambient pressure around the cold trap is delayed with respect to anincrease in the cold panel temperature. Therefore, the standby time maybe set empirically or experimentally considering a delay in the rise ofthe ambient pressure around the cold trap. In the embodiment, thestandby time may be set to several to ten minutes. The controller 24terminates the first temperature raising step after an elapse of thestandby time and performs the first pressure determination process.

The controller 24 performs 260 K temperature control in accordance with,for example, a temperature table shown in FIG. 5. The controller 24determines the operation state of the refrigerator 22 based on thecurrent operation state and the current cold panel temperature T. Morespecifically, the controller 24 determines whether to drive therefrigerator 22 into reverse rotation operation, suspension ofoperation, or normal rotation operation, and determines the operatingfrequency of the refrigerator 22 as well. The controller 24 turns thedetermination into an operation instruction and outputs the same to therefrigerator motor 36. Referring to FIG. 5, notation “-” indicates thatthe current operation state. The same is true of the other diagrams.

According to the temperature table shown in FIG. 5, the controller 24determines the operation state of the refrigerator 22, distinguishingbetween whether the cold panel temperature T falls below the permittedtemperature range, whether the temperature T is within the permittedtemperature range, or whether the temperature T exceeds the permittedtemperature range. The permitted temperature range is defined so as toprovide a predetermined temperature margin above and below the targettemperature of temperature control. In the embodiment, the targettemperature of temperature control is 260 K. Therefore, the permittedtemperature range is defined as 250-260 K, providing a 10 K temperaturemargin. In this example, the target temperature represents the upperlimit temperature of the permitted temperature range. Alternatively, apermitted temperature range may be defined around the target temperaturerange or the target temperature may be the lower limit temperature ofthe permitted temperature range.

If the cold panel temperature T falls below the permitted temperaturerange, i.e., below 250 K (the left column of FIG. 5), the controller 24drives the refrigerator into reverse rotation operation regardless ofthe current operation state of the refrigerator. If the cold paneltemperature T is within the permitted temperature range, i.e., 250K≦T<260 K (the center column of FIG. 5), the operation state iscontinued regardless of the current operation state of the refrigerator.If the cold panel temperature T exceeds the permitted temperature range,i.e., equal to or higher than 260 K (the right column of FIG. 5), thecontroller 24 drives the refrigerator into normal rotation operationregardless of the current operation state of the refrigerator. In thisway, the temperature of the cold panel is raised by reverse rotationoperation if the cold panel temperature T falls below the permittedtemperature range. If the cold panel temperature T exceeds the permittedtemperature range, the cold panel is cooled by normal rotationoperation.

The controller 24 may operate the refrigerator 22 at a higher operatingfrequency in reverse rotation operation than in normal rotationoperation. Preferably, the controller 24 may operate the refrigerator 22at the maximum operating frequency during reverse rotation operation. Inthis way, the cold panel temperature is raised to a target temperature(e.g., the pressure determination temperature) rapidly. In theembodiment, the refrigerator 22 may be operated at the minimum operatingfrequency during normal rotation operation. This is due to the fact thatthe target temperature is low so that the panel temperature is likely todrop. In this regard, instead of driving the refrigerator 22 into normalrotation operation, the operation of the refrigerator 22 may besuspended.

Thus, when 260 K temperature control is started in the first temperatureraising step according to the embodiment, the temperature of the coldpanel 20 is rapidly raised to 260k, which is the pressure determinationtemperature, by driving the refrigerator 22 into reverse rotationoperation to produce the maximum output. When the temperature is raisedto 260 K, the refrigerator 22 is switched to normal rotation operation,cooling the panel to 250 K. When the temperature reaches 250 K, thetemperature is raised to 260 K again. The steps will be repeated untilthe standby time elapses.

FIG. 6 is a flowchart illustrating the second temperature raising step(S16 of FIG. 3) according to the embodiment. If it is determined in thefirst pressure determination process (S12 of FIG. 3) that the ambientpressure around the cold trap is equal to lower than the referencepressure (No in S12 of FIG. 3), the controller 24 starts temperaturecontrol wherein the first regeneration temperature is the targettemperature (S40). Such temperature control may be referred to ashigh-temperature control hereinafter. Since the first regenerationtemperature is 320 K in the embodiment, such temperature control may bereferred to as 320 K temperature control.

The controller 24 continues 320 K temperature control for apredetermined standby time. The controller 24 determines whether thestandby time has elapsed since the start of 320 K temperature control(S42). The standby time is provided because it is considered that anincrease in the ambient pressure around the cold trap is delayed withrespect to an increase in the cold panel temperature. Therefore, thestandby time may be set empirically or experimentally considering adelay in the rise of the ambient pressure around the cold trap. In theembodiment, the standby time may be set to several to minutes.

If it is determined that the standby time has not elapsed (No in S42),the controller 24 determines whether the standby time has elapsed at asubsequent point of time in the control schedule (S42). If it isdetermined that the standby time has elapsed (Yes in S42), thecontroller 24 performs the second pressure determination process (S18)of FIG. 3). Since 320 K temperature control raises the temperaturerelatively slowly, the cold panel temperature may not have reached 320 Kwhen the standby time has elapsed.

The controller 24 performs 320 K temperature control in accordance withcontrol tables shown in FIGS. 7 through 9. FIG. 7 shows a pressure tablefor 320 K temperature control and FIG. 8 shows a temperature table for320 K temperature control. FIG. 9 shows a final output table fordetermining a final output. In 320 K temperature control, the controller24 uses the temperature table and the pressure table so as to output theoperation state of the refrigerator 22. The controller 24 then uses thefinal output table so as to determine one of the output from thetemperature table and the output from the pressure table as the finaloutput. The controller 24 controls the refrigerator 22 in accordancewith the final output thus determined.

According to the pressure table shown in FIG. 7, the controller 24determines the operation state of the refrigerator 22, distinguishingbetween whether the ambient pressure P around the cold trap falls belowthe permitted pressure range, whether the pressure P is within thepermitted pressure range, or whether the pressure P exceeds thepermitted pressure range. For example, the upper limit of the permittedpressure range is set to be equal to or lower than the limit pressure.The lower limit of the permitted pressure range may be set to be lowerthan the upper limit by a predetermined pressure margin. Since the limitpressure is 100 Pa according to the embodiment, the upper limit and thelower limit of the permitted pressure range are set to 100 Pa and 80 Pa,respectively.

If the ambient pressure P around the cold trap falls below the permittedpressure range, i.e., below 80 Pa (the left column of FIG. 7), thecontroller 24 drives the refrigerator into reverse rotation operationregardless of the current operation state of the refrigerator. If theambient pressure P around the cold trap is within the permitted pressurerange, i.e., 80 Pa≦P<100 Pa (the center column of FIG. 7), the operationstate is continued regardless of the current operation state of therefrigerator. If the ambient pressure P around the cold trap exceeds thepermitted pressure range, i.e., equal to or higher than 100 Pa (theright column of FIG. 7), the controller 24 drives the refrigerator intonormal rotation operation regardless of the current operation state ofthe refrigerator. In this way, the temperature of the cold panel israised and the pressure is increased by reverse rotation operation ifthe ambient pressure P around the cold trap falls below the permittedpressure range. If the ambient pressure P around the cold trap exceedsthe permitted pressure range, the cold panel is cooled and the pressureis decreased by normal rotation operation.

In normal rotation operation of the refrigerator 22 performed when theambient pressure P around the cold trap exceeds the permitted pressurerange, the controller 24 uses the predetermined standby temperature asthe target temperature. The standby temperature is selected from thenon-liquefaction temperature range. In the embodiment, the standbytemperature is set to 260 K. In normal rotation operation in this case,the controller 22 operates the refrigerator 22 at the maximum operatingfrequency. This causes the cold panel temperature T to drop rapidly,returning the ambient pressure P around the cold trap to the permittedpressure range efficiently.

When the cold panel temperature T sufficiently approximates the standbytemperature 260 K or has reached 260 K, the controller 24 places therefrigerator 22 in the standby operation state. For example, theoperating frequency of the refrigerator 22 in the standby operationstate is selected such that the heat load on the cold trap 10 and therefrigerating capacity of the refrigerator 22 balance and the cold paneltemperature T is maintained at the standby temperature accordingly. Forexample, the refrigerator may be operated in the standby operation stateat the minimum operating frequency that allows the refrigerator 22 to beoperated stably.

The pressure table shown in FIG. 7 does not output “suspension ofoperation” as the operation state of the refrigerator 22. This is due tothe fact that, it is preferable to cool the cold panel 20 actively bynormal rotation operation of the refrigerator 22 instead of naturalcooling induced by suspension of operation of the refrigerator 22, whenthe ambient pressure P around the cold trap is increased.

According to the temperature table shown in FIG. 8 the controller 24determines the operation state of the refrigerator 22, distinguishingbetween whether the cold panel temperature T falls below the permittedtemperature range, whether the temperature T is within the permittedtemperature range, or whether the temperature T exceeds the permittedtemperature range. The permitted temperature range is configuredsimilarly to the temperature table for 260 K temperature control shownin FIG. 5. In 320 K temperature control, the target temperature is 320 Kso that the permitted temperature range is defined as 310≦T<320 K.

If the cold panel temperature T falls below the permitted temperaturerange, i.e., below 310 K (the left column of FIG. 8), the controller 24drives the refrigerator into reverse rotation operation regardless ofthe current operation state of the refrigerator. If the cold paneltemperature T is within the permitted temperature range, i.e., 310K≦T<320 K (the center column of FIG. 8), the operation state iscontinued regardless of the current operation state of the refrigerator.If the cold panel temperature T exceeds the permitted temperature range,i.e., equal to or higher than 320 K (the right column of FIG. 8), thecontroller 24 suspends the operation of the refrigerator regardless ofthe current operation state of the refrigerator. In this way, thetemperature of the cold panel is raised by reverse rotation operation ifthe cold panel temperature T falls below the permitted temperaturerange. If the cold panel temperature T exceeds the permitted temperaturerange, the cold panel is naturally cooled as a result of the suspensionof operation.

The pressure table shown in FIG. 8 does not output “normal rotationoperation” as the operation state of the refrigerator 22. Since the coldpanel temperature T is higher than the ambient temperature, the coldpanel 20 is naturally cooled as a result of the suspension of operation.Thus, the panel need not be actively cooled by normal rotationoperation.

The final output table shown in FIG. 9 indicates that, when the outputfrom the pressure table is “normal rotation operation”, the controller24 determines the output from the pressure table as the final outputregardless of the output from the temperature table. This ensures thatthe refrigerator 22 is placed in the state of normal rotation operationwhen the ambient pressure P around the cold trap exceeds the permittedpressure range. As a result, the ambient pressure P around the cold trapis returned to the permitted pressure range. Meanwhile, when the outputfrom the pressure table is “reverse rotation operation” and the outputfrom the temperature table is “suspension of operation”, the controller24 determines “suspension of operation”, i.e., the output from thetemperature table, as the final output. That the temperature tableoutputs “suspension of operation” means that the cold panel temperatureT is higher than the permitted temperature range and that there is noneed to raise the temperature. When the outputs from both the pressuretable and the temperature table are “reverse rotation operation”, thecontroller 24 determines “reverse rotation operation” as the finaloutput.

Thus, when 320 K temperature control is started in the secondtemperature raising step according to the embodiment, the temperature ofthe cold panel 20 is raised to 320 k, which is the first regenerationtemperature, by driving the refrigerator 22 into reverse rotationoperation. Since the operating frequency of the refrigerator 22 in thiscase is lower than the operating frequency during reverse rotationoperation in 260 K temperature control, the temperature is raisedrelatively slowly. When the temperature is raised to 320 K, theoperation of the refrigerator 22 is suspended, naturally cooling thepanel to 310 K. When the temperature reaches 310 K, the temperature israised to 320 K again. The steps will be repeated so long as the ambientpressure P around the cold trap remains within the permitted pressurerange, until the completion of the high-temperature pumping step.

However, when the ambient pressure P around the cold trap exceeds thepermitted pressure range, the refrigerator 22 is driven to produce themaximum output so as to cool the cold panel 20 rapidly to 260 K, whichis the standby temperature. Since the standby temperature is within thenon-liquefaction temperature range, the gas frozen on the panel is notliquefied or liquefaction is limited to the minimum degree, even if theambient pressure P around the cold trap exceeds the permitted pressurerange. The cold panel temperature T is maintained at the standbytemperature until the ambient pressure P around the cold trap fallsbelow the permitted pressure range. After the ambient pressure returnsto the permitted pressure range, normal 320 K temperature control isresumed so that the cold panel temperature T is raised by reverserotation operation to 320 K, which is the first regenerationtemperature.

FIG. 10 is a flowchart illustrating the high-temperature pumping step(S22 of FIG. 3) according to the embodiment. In the high-temperaturepumping step, 320 K temperature control is continued as in the secondtemperature raising step. The controller 24 determines whether a maximumpumping time has elapsed (S50) and determines whether the ambientpressure around the cold trap is lower than the pumping completionpressure (S52). Whichever of these steps of determination may precedethe other. When the maximum pumping time has not elapsed (No in S50) andwhen the ambient pressure around the cold trap is equal to or higher thepumping completion pressure (No in S52), the controller 24 repeats thesesteps of determination at a subsequent point of time in the controlschedule.

When it is determined that the maximum pumping time has elapsed (Yes inS50), the controller 24 terminates the high-temperature pumping step andperforms a cool-down step (S24 of FIG. 3). The maximum pumping time is amaximum value of time permitted for a high-temperature pumping step andis predefined by and stored in the controller 24. When the maximumpumping time has elapsed, the regeneration process is terminatedregardless of whether the ice on the cold panel is completelydischarged. Normally, the maximum pumping time is set to be sufficientto completely discharge the ice on the cold panel.

If it is determined that the ambient pressure around the cold trap islower than the pumping completion pressure (Yes in S52), the controller24 stands by for a predetermined period of time (S54). The standby timeis predefined by and stored in the controller 24. For example, thestandby time is set to, for example, several minutes. After the standbytime has elapsed, the controller 24 determines for a second time whetherthe ambient pressure abound the cold trap is lower than the pumpingcompletion pressure (S56). By performing a determination as to thepumping completion pressure after the standby time has elapsed, apremature determination of completion of pumping, which results from aninstantaneous drop in the pressure at the time of initial determinationas to the pumping completion pressure (S52), can be avoided. If it isdetermined that the ambient pressure around the cold trap is equal to orhigher than the pumping completion pressure in the re-determination (Noin S56), the controller 24 stands by for a predetermined period of timeagain (S54) and performs the determination as to the completion ofpumping (S56). If it is determined that the ambient pressure around thecold trap is equal to or lower than the pumping completion pressure inthe re-determination (Yes in S56), the controller 24 terminates thehigh-temperature pumping step and performs a cool-down step (S24 of FIG.3).

The low-temperature step (S14 of FIG. 3) is performed similarly to theabove-mentioned high-temperature pumping step. A difference is that thelow-temperature pumping step is performed under 260 K temperaturecontrol. Another difference is that the maximum pumping time in thelow-temperature pumping step is set to be longer than that of thehigh-temperature pumping step.

FIG. 11 is a flowchart illustrating the cool-down step (S24 of FIG. 3)according to the embodiment. In the cool-down step, the controller 24starts controlling the temperature of the cold trap 10, targeting apumping operation temperature (S60). The controller 24 determines theoperating frequency of the refrigerator 22 so as to minimize an errorbetween the pumping operation temperature and the cold paneltemperature. Since the pumping operation temperature is lower than theregeneration temperature, the cold trap 10 is cooled. The pumpingoperation temperature is, for example, 100 K.

The controller 24 determines whether the cold panel temperature hasreached a zero-point adjustment temperature of the pressure sensor 40(S62). If it is determined that the cold panel temperature has notreached the zero-point adjustment temperature of the temperature sensor40 (No in S62), the controller 24 performs the determination again at asubsequent point of time in the control schedule (S62). If it isdetermined that the cold panel is cooled to the zero-point adjustmenttemperature of the pressure sensor 40 (Yes in S62), the controller 24performs zero-point adjustment of the pressure sensor 40 (S64).

Further, the controller 24 determines whether the cold panel temperatureT has reached a cool-down completion temperature (S66). If it isdetermined that the cold panel temperature has not reached the cool-downcompletion temperature (No in S66), the controller 24 performs thedetermination again at a subsequent point of time in the controlschedule (S66). If it is determined that the cold panel is cooled to thecool-down completion temperature (Yes in S66), the controller 24determines that the regeneration process is completed and terminates theregeneration process (S68).

The cool-down completion temperature is set to be equal to, for example,the above-mentioned pumping operation temperature. The zero-pointadjustment temperature is set to be higher than the cool-down completiontemperature by a predetermined margin so that zero-point adjustment iscompleted before the cool-down step is terminated. Zero-point adjustmentof the pressure sensor may be performed in the cool-down step withoutexception, or performed at appropriate intervals but not each time thecool-down step is performed, or totally omitted. Alternatively,zero-point adjustment may be performed when the ambient pressure aroundthe cold trap is determined to be lower than the pumping completionpressure and the cool-down step is then started accordingly.

According to the regeneration method of the embodiment, the ambientpressure is controlled so that the gas captured by a cold trap is notmelted at a relatively high regeneration temperature at which the gascan be melted. In this way, regeneration time is considerably reduced ascompared with the related-art low-temperature regeneration. For example,it was experimentally verified that, a regeneration time of 160 minutesrequired in low-temperature regeneration under 260 K temperature controlis reduced by about half to 85 minutes, if the regeneration methodaccording to the embodiment is used in place of low-temperatureregeneration. FIG. 12 shows the time variation of the cold paneltemperature and ambient pressure around the cold trap.

In the exemplary embodiment shown in FIG. 12, the above-mentionedregeneration process was performed by setting the upper limit and thelower limit of the permitted pressure range of the ambient pressurearound the cold trap to 100 Pa and 80 Pa, respectively. The verticalaxis of the graph of FIG. 12 indicates the cold panel temperature andambient pressure around the cold trap, and the horizontal axis indicatestime elapsed since the start of the regeneration process.

Referring to FIG. 12, an interval between the start of regeneration anda point of time B marks the first temperature raising step (S10 of FIG.3, and FIG. 4). When the regeneration process is started, therefrigerator 22 is driven into reverse rotation operation to produce themaximum output so that the cold panel temperature is rapidly raised tothe pressure determination temperature (260 K). At a point of time A,the cold panel temperature reaches 260 K. Subsequently, the cold paneltemperature is maintained within the permitted temperature range 250K≦T<260 K in accordance with the temperature table for 260 K temperaturecontrol (FIG. 5) until the standby time elapses (S36 of FIG. 4).

At the point of time B when the standby time has elapsed, the firsttemperature raising step is terminated and the first pressuredetermination process (S12 of FIG. 3) is performed. At the point of timeB, the ambient pressure around the cold trap is still about 0 Pa, whichis lower than the reference pressure (100 Pa). Therefore, the secondtemperature raising step is then performed (S16 of FIG. 3, FIG. 6). Aninterval between the point of time B and a point of time C marks thesecond temperature raising step. In the second temperature raising step,the temperature of the cold panel 20 is raised at a rate slower thanthat of the first temperature raising step. During the secondtemperature raising step, the ambient pressure around the cold trapstarts to rise. As illustrated, an increase in the ambient pressurearound the cold trap is delayed with respect to an increase in the coldpanel temperature.

At the point of time C when the standby time (S42 of FIG. 6) haselapsed, the second temperature raising step is terminated and thesecond pressure determination process (S18 of FIG. 3) is performed. Atthe point of time C, the cold panel temperature is about 290 K and hasnot reached the target temperature (320 K) of the second temperatureraising step. At the point of time C, the ambient pressure around thecold trap is about 25 Pa, which is higher than the pumping completionpressure (5 Pa). Therefore, the high-temperature pumping step is thenperformed (S22 of FIG. 3, FIG. 10).

An interval between the point of time C and a point of time E marks thehigh-temperature pumping step. In the high-temperature pumping step, 320K temperature control is performed as in the second temperature raisingstep. Thus, the operation state of the refrigerator 22 is determined inaccordance with the pressure table, the temperature table, and the finaloutput table shown in FIGS. 7 through 9 so that the cold paneltemperature is controlled accordingly. In the exemplary embodiment, theambient pressure around the cold trap is about 90 Pa at maximum and doesnot exceed the upper limit of the permitted pressure range. The finaloutput table (FIG. 9) continues to indicate that the output from thetemperature table (FIG. 8) for 320 K temperature control is the finaloutput until the ambient pressure around the cold trap is lower than thepumping completion pressure (5 Pa) (S52 of FIG. 10) at a point of timeD. Thereby, the cold panel temperature is maintained at the permittedtemperature range 310 K≦T<320 K.

At the point of time E when the standby time (S54 of FIG. 10) haselapsed, the ambient pressure around the cold trap is still lower thanthe pumping completion pressure (S56 of FIG. 10). Therefore, thehigh-temperature pumping step is terminated and the cool-down step (S24of FIG. 3, FIG. 11) is performed at the point of time E. In thecool-down step, the refrigerator 22 is driven into normal rotationoperation to produce the maximum output so that the cold pane is rapidlycooled to the pumping operation temperature (100 K). At a point of timeF, the cold panel temperature reaches 100 K, whereupon the regenerationprocess is terminated. The regeneration process is completed in about 85minutes, which is about half the time required in the related art.

FIG. 13 is a graph showing another example of the time variation of thetemperature and pressure in the regeneration process according to theembodiment. In the exemplary embodiment shown in FIG. 13, theabove-mentioned regeneration process was performed by setting the upperlimit and the lower limit of the permitted pressure range of the ambientpressure around the cold trap to 80 Pa and 60 Pa, respectively. Theexemplary embodiment shown in FIG. 13 is the same as the exemplaryembodiment shown in FIG. 12 except for the permitted pressure range. Asin FIG. 12, the vertical axis of the graph of FIG. 13 indicates the coldpanel temperature and ambient pressure around the cold trap, and thehorizontal axis indicates time elapsed since the start of theregeneration process.

The exemplary embodiment shown in FIG. 13 differs from the exemplaryembodiment shown in FIG. 12 in that the panel is temporarily cooled tothe standby temperature in the high-temperature pumping step. Referringto FIG. 13, an interval between a point of time G and a point of time Kmarks the high-temperature pumping step. In the interval between thepoint of time G and a point of time H, the cold panel temperature israised to the first regeneration temperature (320 K). In this process,the output from the temperature table (FIG. 8) for 320 K temperaturecontrol is indicated by the final output table (FIG. 9) as being thefinal output so that the temperature of the cold panel 20 is raised byreverse rotation operation of the refrigerator 22.

At the point of time H, the ambient pressure around the cold trapreaches 80 Pa, which is the upper limit pressure. For this reason, theoutput from the pressure table (FIG. 7) for 320 K temperature control isindicated by the final output table as being the final output (FIG. 9)so that the operation state of the refrigerator 22 is switched to normalrotation operation and the cold panel is rapidly cooled to the standbytemperature (260 K). At a point of time I, when the cold panel is cooledto the standby temperature, the refrigerator 22 is placed in the standbyoperation state so that the cold panel is maintained at the standbytemperature. Subsequently, at a point of time J, the ambient pressurearound the cold panel reaches 60 Pa, which is the lower limit pressure.At this point of time, the cold panel is at the standby temperature,which falls below the permitted temperature range. Thus, the pressuretable (FIG. 7) for 320 K temperature control outputs “reverse rotationoperation” and the temperature table (FIG. 8) for 320 K temperaturecontrol also outputs “reverse rotation operation”. Accordingly, theoperation state of the refrigerator 22 is switched to reverse rotationoperation so that the temperature is raised again to the firstregeneration temperature. At the point of time K, the ambient pressurearound the cold trap is lower than the pumping completion pressure sothat the high-temperature pumping step is terminated. The exemplaryembodiment shown in FIG. 13 requires a regeneration time of about 93minutes, which is also remarkably shorter than the time required in therelated-art low-temperature regeneration. In yet another variation, theupper limit pressure and the lower limit pressure of the permittedtemperature range are lowered to 60 Pa and 40 Pa, respectively.Regeneration was also completed in about 93 minutes.

As described, according to the embodiment, regeneration time isdramatically reduced as compared to the related-art low-temperatureregeneration, by regenerating the cold trap 10 while monitoring thetemperature and pressure. Since the temperature and pressure arecontrolled so that the gas frozen on the cold trap 10 is not meltedduring regeneration, liquid is prevented from dropping from the coldtrap 10 onto other equipment such as the turbomolecular pump 12,ensuring that the equipment is not adversely affected. For example, theinventive regeneration prevents damage to equipment caused when thefrozen gas is melted and solid objects as well as liquid drop onto theequipment.

1. A cold trap provided in a pumping path connecting a volume subject topumping to a vacuum pump, causing a portion of a gas taken in from thevolume to the vacuum pump via the pumping path to be frozen on thesurface of the cold trap, and capturing the gas accordingly, the coldtrap comprising: a cold panel provided in the pumping path such that thepanel is exposed; a refrigerator thermally coupled to the cold panel andoperative to cool the cold panel; and a controller configured, in aregeneration process for evaporating a gas frozen on the surface of thecold panel and discharging the gas outside using the vacuum pump, tocontrol the refrigerator so as to raise the temperature of the coldpanel to a temperature exceeding a non-liquefaction temperature rangeand to adjust, at the temperature exceeding the non-liquefactiontemperature range, a pressure in the pumping path so that the gas frozenon the surface of the cold panel is evaporated without being melted, thenon-liquefaction temperature range being a range in which it isguaranteed that a gas frozen on the surface of the cold trap isevaporated without being melted.
 2. The cold trap according to claim 1,wherein the controller controls the refrigerator so that the pressure inthe pumping path does not exceed the pressure at the triple point of thegas frozen on the cold panel.
 3. The cold trap according to claim 1,wherein the controller controls the refrigerator so that the pressure inthe pumping path does not exceed a permissible inlet pressure of thevacuum pump.
 4. The cold trap according to claim 1, wherein when thepressure in the pumping path exceeds an upper limit pressure, thecontroller returns the pressure in the pumping path to a pressure equalto or below the upper limit pressure by cooling the cold panel.
 5. Thecold trap according to claim 1, wherein when the pressure in the pumpingpath exceeds a permitted pressure range, the controller cools the coldpanel to a standby temperature selected from the non-liquefactiontemperature range.
 6. The cold trap according to claim 5, wherein whenthe pressure in the pumping path falls below the permitted pressurerange at the standby temperature, the controller raises the temperatureof the cold panel to a temperature exceeding the non-liquefactiontemperature range.
 7. The cold trap according to claim 1, wherein thecontroller raises the temperature of the cold panel at a slower rate ata temperature exceeding the non-liquefaction temperature range than at atemperature within the non-liquefaction temperature range.
 8. The coldtrap according to claim 1, wherein the controller raises the temperatureof the cold panel to a pressure determination temperature selected fromthe non-liquefaction temperature range and determines, after thetemperature is raised, whether the pressure in the pumping path exceedsa reference pressure, and when it is determined that the pressure in thepumping path exceeds the reference pressure, the controller cools thecold panel to a temperature selected from the non-liquefactiontemperature range, and, when it is determined that the pressure in thepumping path does not exceed the reference pressure, the controllerraises the temperature of the cold panel to a temperature exceeding thenon-liquefaction temperature range.
 9. The cold trap according to claim1, further comprising: a pressure sensor that includes in itsmeasurement range the whole pressure range that could occur in thepumping path in the regeneration process, is provided to measure thepressure in the pumping path, and is connected to the controller so asto output a measured value to the controller, wherein the controllercontrols the pressure in the pumping path based on the measured valuefrom the pressure sensor.
 10. A cold trap regeneration method forevaporating ice captured on the surface of a cold trap and dischargingthe ice outside, the method comprising: raising the temperature of thecold trap to a temperature exceeding a non-liquefaction temperaturerange in which it is guaranteed that ice frozen on the surface of thecold trap is evaporated without being melted; and controlling an ambientpressure around the cold trap at the temperature exceeding thenon-liquefaction temperature range so that the ice frozen on the surfaceof the cold trap is evaporated without being melted.
 11. Theregeneration method according to claim 10, wherein the controllingexercises control so that the ambient pressure around the cold trap doesnot exceed the pressure at the triple point of water.
 12. Theregeneration method according to claim 10, wherein the controllingincludes discharging water vapor outside using a turbomolecular pump andcontrolling the ambient pressure around the cold trap so as not toexceed a permissible inlet pressure of the turbomolecular pump.
 13. Theregeneration method according to claim 10, wherein when the ambientpressure around the cold trap exceeds an upper limit pressure, thecontrolling returns the ambient pressure to a pressure equal to or belowthe upper limit pressure by cooling the cold trap.
 14. The regenerationmethod according to claim 10, wherein when the ambient pressure aroundthe cold trap exceeds a permitted pressure range, the controlling coolsthe cold trap to a standby temperature selected from thenon-liquefaction temperature range.
 15. The regeneration methodaccording to claim 14, wherein when the ambient pressure around the coldtrap falls below the permitted pressure range at the standbytemperature, the controlling raises the temperature of the cold trap toa temperature exceeding the non-liquefaction temperature range.
 16. Theregeneration method according to claim 10, wherein the raising raisesthe temperature of the cold trap at a slower rate at a temperatureexceeding the non-liquefaction temperature range than at a temperaturewithin the non-liquefaction temperature range.
 17. The regenerationmethod according to claim 10, wherein the raising raises the temperatureof the cold trap to a pressure determination temperature selected fromthe non-liquefaction temperature range and determines, after thetemperature is raised, whether the ambient pressure exceeds a referencepressure, and when it is determined that the ambient pressure exceedsthe reference pressure, the ice is allowed to sublimate at a temperatureselected from the non-liquefaction temperature range and is dischargedoutside, and, when it is determined that the ambient pressure does notexceed the reference pressure, the temperature of the cold trap israised to a temperature exceeding the non-liquefaction temperaturerange.
 18. The regeneration method according to claim 10, wherein asingle pressure sensor is operative to measure the ambient pressurearound the cold trap since the start of the raising through thecompletion of the controlling.
 19. A regeneration controller adapted toperform a regeneration process for evaporating a gas frozen on thesurface of a cold trap and discharging the gas outside, wherein thecontroller is operative to raise the temperature of the cold trap to atemperature exceeding a non-liquefaction temperature range and tocontrol an ambient pressure around the cold trap at the temperature sothat the gas frozen on the surface of the cold trap is evaporatedwithout being melted, the non-liquefaction temperature range being arange in which it is guaranteed that a gas frozen on the surface of thecold trap is evaporated without being melted.
 20. A cold trapregeneration method for evaporating a gas frozen on the surface of acold trap and discharging the gas outside, the method comprising:monitoring an ambient pressure around the cold trap during regeneration;and temporarily cooling the cold trap when the ambient pressure thusmonitored exceeds a permitted pressure range.